In view of recent growing interest in global warming and of environmental regulation enforcement, it is an urgent problem to reduce the exhaust gas emission from coal power plants. As the means for reduction of the exhaust gas emission, not only exhausting reduction by improving the efficiency of the plants but also CO2 recovery by chemical absorbents is now attracting attention of the people.
As the chemical absorbents, amine compounds have long been studied on its CO2 absorption. For example, carbon dioxide is introduced into an absorption tower, where the carbon dioxide is brought into contact with an alkanolamine aqueous solution alone or with a reaction accelerator such as piperazine so as to be absorbed therein; and thereafter the solution is heated to separate and recover the carbon dioxide in a desorption tower. Examples of the alkanolamine include monoethanolamine (hereinafter, referred to as “MEA”) and methyl diethanolamine (hereinafter, referred to as “MDEA”).
However, there are a lot of problems. For example, if the alkanolamine is so oxidized and degraded that the absorbing solution becomes acidic, the solution loses alkalinity, which plays a vital role in absorbing carbon dioxide. Further, the degradation products may corrode carbon steel used in the absorption tower.
To solve those problems, an attempt is made (for example, in Patent document 1) to incorporate an anti-oxidizing agent into the reaction system so as to reduce the oxidization and degradation of the absorbing solution. Specifically, the document discloses an invention in which a free radical scavenger such as a thiosulfate is added as the anti-oxidizing agent into the reaction system to slow down the degradation rate of MEA.
However, the document is silent about effective measures against N,N-bis-(2-hydroxyethyl)glycine (hereinafter, referred to as “BICINE”), which is an oxidization degradation product of alkanolamine and which is presumed to corrode the carbon steel. Accordingly, there still remains a problem on preventing the degradation of the absorbing solution.